In order to achieve high definition and large area in liquid crystal display, a high-multiplex operation display in which the twist angle of liquid crystal molecules was controlled greater than conventional 90 degrees has been put to practical use. This is generally called a supertwisted nematic mode (STN-type mode), and an SBE mode, an STN mode and the like are known. In this STN-type mode, steep deformation of molecular orientation by application of voltage is combined with the optical effect of birefringence to optimize retardation (a product of the refractive index anisotropy of a liquid crystal and a cell gap=.DELTA.n.d) and the azimuthal angle of a polarizer so as to achieve still better display characteristics. In recent years, black-and-white display has come to be achieved in the STN mode by a system in which a phase plate is used to compensate for a phase difference of transmitted light, which has been caused by the birefringence effect, or the like. It is also possible to form full-color images by supplementing color filters, as needed.
Incidentally, such a phase plate is a plate used in shifting the relative phase of a polarized component of light and made of a birefringent material. In the plate, an oriented film made of a synthetic resin is used as a birefringent layer. Examples of the structure of the phase plate include a single-layer structure composed of one birefringent layer, a multi-layer structure obtained by laminating two or more birefringent layers which are identical or different in birefringent behavior, a structure with a protective layer, and the like (for example, Japanese Patent Application Laid-Open No. 158701/1990).
In order to provide bright colors and high-definition images, for example, a phase plate for liquid crystal displays is required that its birefringent layer is optically even at the whole surface and undergoes no change in optical properties even by changes of temperature and/or humidity. In particular, if the phase plate is used in a liquid crystal display panel for installation in a car, it is required to withstand a temperature of at least 60.degree. C. or higher, preferably 80.degree. C. or higher, more preferably 100.degree. C. or higher because it is predicted that the panel would be used under severe conditions.
As synthetic resin materials for such phase plates, there have heretofore been used various kinds of film-forming resins such as phenoxyether type crosslinkable resins, epoxy resins, polycarbonate resins, allylate resins and polycarbonate resins.
However, the phenoxyether type crosslinkable resins, epoxy resins, polycarbonate resins, allylate resins and the like are difficult to stretch evenly, and moreover are insufficient in moisture resistance, and hence have hygroscopicity of the order of 0.1-0.2 wt. %. Therefore, their retardation stability is lowered by changes of humidity in a service environment. Besides, since the polycarbonate resins, allylate resins and the like have high heat resistance, their stretching temperature is high. Therefore, it is hard to control the stretching temperature, and hence to produce an optically even phase plate.
A film obtained by stretching and orienting a sheet produced by solvent casting is excellent in surface smoothness, but involves problems that it is poor in productivity, and may not be used according to its service environment because a solvent remains therein. Therefore, an extrusion process making use of a T-die, or the like is used in the polycarbonate resins and the like. However, if a sheet having a thickness of 100-200 .mu.m is formed, the unevenness of thickness can be controlled only to the extent of 6-8 .mu.m.
In addition, since these oriented, synthetic resin films generally have a photoelastic coefficient as great as 50-100.times.10.sup.-13 cm.sup.2 /dyne, there is a problem that their retardation values are changed greatly by a slight stress. Besides, the unevenness of thickness which occurs in the sheets before stretching directly influences the scattering of retardation after stretching.
As described above, the phase plates composed individually of the oriented, synthetic resin films conventionally known are not fully satisfactory for liquid crystal displays. There is hence demand for improving them.